This invention relates to a method of enhancing the purity of a body of fused quartz, e.g. vitreous silica, having opposed boundary surfaces, the method being of the kind, which includes the steps of comprising maintaining the body at a temperature above 1000.degree. C. and at the same time applying a polarizing potential across the boundary surfaces by electrodes in contact with the boundary surfaces so that at least some of the residual impurity ions are made to migrate away from one boundary surface towards the opposite boundary surface thereof and are subsequently discharged into the gaseous phase at the latter boundary surface. In particular the invention is concerned with an electrical purification method and articles purified by the method.
High purity vitreous silica (fused quartz) articles are commonly made by flame or electrical fusion of carefully refined powders derived from natural quartz crystal. With increasing attention being paid to residual impurities by certain industries, e.g. semiconductor and optical fibre manufacture, the level of purities sought is at times beyond that which can be achieved by conventional chemical and physical refinement techniques. There may even be an undesirable concentration of alkali ions, in articles manufactured from synthetic quartz or amorphous silica powders, either derived from the starting material or introduced accidentally as contamination in the manufacturing process.
For achieving very low levels of certain mobile metal ion impurities, notably ions of the alkali metals, e.g. lithium, sodium, potassium and copper, and the application of an electrical polarizing potential across the wall of a heated quartz article has been proposed (GB-A-2166434). This can effect migration of the impurity ions away from one of the wall surfaces, and toward the opposite wall surface. The article may subsequently be cooled to ambient temperature while the polarizing potential is still applied, in which case the impurities accumulating near the cathodic wall surface may be removed by machining or etching off a thin layer at that surface to leave the remaining mass of vitreous silica of higher average purity.
In one example included in the above-noted patent application, a method was described in which a vitreous silica crucible was inverted over an internal graphite mould and the crucible was rotated slowly while being heated wtih an oxy-propane burner. An electric potential of 4.5 kV was applied with the mould serving as negative electrode and the burner flame as positive electrode. Significant depletion of alkali metal ions was observed in the inner layers of the crucible, but while some coloration was observed in the flame due to the release of ions electrolyzed from the crucible into the gas flame, significant amounts of alkali metal remained in the outer layers of glass.